The development of new methods to use renewable, non-polluting energy sources is essential to meet future needs for energy. Energy from the sun is one of the energy sources which are of the greatest interest in this context.
Silicon is a critical raw material for both the electronics industry and the solar cell industry. Although there are alternative materials for specific applications, multi-crystalline and mono-crystalline silicon will be the material of choice for the foreseeable future. Improved availability and economics of production of multi-crystalline silicon will increase the growth possibilities for both these industries, in particular the application of solar cells for renewable energy.
Currently, to manufacture silicon of a satisfactory purity for use in solar cells or electronics, chemical vapour deposition methods (CVD—Chemical Vapour Deposition) are primarily used. Different embodiments of the Siemens process are the most used forms of CVD for the manufacture of polycrystalline silicon. In this method, silicon containing gases, such as silane or trichlorosilane, and other gases such as hydrogen and argon, are fed into a cooled container and silicon is deposited on one or more resistance-heated rods. The process is very energy and labour demanding. A more detailed description of the most used process can be found in the U.S. Pat. No. 3,979,490.
Another CVD method uses a fluidized bed, whereby silicon seed particles are fluidised and held in an upward flowing gas stream, with the gas stream comprising silicon-containing gas from which silicon can be deposited onto the seed particles. A full description of CVD methods with a fluidised bed and associated equipment and operating parameters for the manufacture of silicon, including gas mixtures, temperatures for the deposition and related problems and limitations can be found in the U.S. Pat. No. 4,818,495 and U.S. Pat. No. 5,810,934, and reference is given to these publications for more information.
A rotating CVD reactor has been developed and a patent has been applied for by Dynatec Engineering with the patent applications NO 2009 2111 and NO 2010 0210. A reactor for the manufacture of silicon by chemical vapour deposition is thus provided, with the reactor comprising a reactor body that forms a container, with at least one inlet for a silicon-containing gas, at least one outlet and at least one heat appliance as a part of, or operatively arranged to, the reactor. In one main embodiment the reactor is characterised in that it rotates so that the reaction gas is subjected to centripetal acceleration. In the following, such a reactor is designated as a Dynatec reactor.
The principle for a Dynatec reactor is that a silicon-containing gas, preferably silane, is fed into a rotating, heated container. The difference in density between the reaction gas and the residual gas makes it possible to separate the gases with the help of the centripetal acceleration. The heavy reaction gas is forced out from the centre of the container, whereupon it decomposes when it is heated up by the inner wall of the container, whereupon silicon is deposited. This gives a higher rate of deposition and a better utilisation of the reaction gas, at the same time as the need for selective cooling of surfaces where the decomposition shall not occur is minimised. Together, this reduces the energy consumption per kilo produced silicon. However, there is a need for further improvement of the Dynatec reactor to lower the cost per produced kg super-clean silicon further and the aim of the present invention is to provide a such improvement.